The present invention relates to a hydraulic control system for an automatic transmission.
Automatic transmissions are known wherein one friction unit is engaged and another friction unit is disengaged to effect a shifting between speed ratios, i.e., between drive ratios. In operating such automatic transmissions, if the engagement of the one friction unit does not take place in good timed relationship with the disengagement of the other friction unit, substantial shocks will take place or the engine will race. It is therefore desired that the engagement and disengagement timing is set to be optimum over all of running states of an automotive vehicle. An automatic transmission is known wherein one friction unit is engaged during operation with two speed ratios. If the engagement of this friction unit is effected in the same timing in shifting into a first one of the two speed ratios from another speed ratio as in shifting from a second one of the two speed ratios from the other speed ratio, optimum performance in each of the shifting operations can not be realized. Taking, for example, an automatic transmission wherein a clutch is engaged during operation with the second and third speed ratios and released during operation with the fourth speed ratio, if the clutch is engaged in the same timing in downshifting from the fourth speed ratio to the second speed ratio as in downshifting from the fourth speed ratio to the third speed ratio, the optimum performance in each of the downshifting is hardly obtained because a difference between a change in engine torque and a change in engine revolution speed before and after the shifting are different in the case of 4-2 downshifting from those in the case of 4-3 downshifting. That is, if, with the optimum timing for the 4-3 downshifting, the clutch is engaged in 4-2 downshifting, substantial shocks will take place. On the contrary, if, with the optimum timing for the 4-2 downshifting, the clutch is engaged in 4-3 downshifting, the engine will race. Therefore, a problem resides in that optimum shifting operation cannot be obtained in each of downshifting from n+2nd speed ratio to n+1st speed ratio and downshifting from the n+2nd speed ratio to nth speed ratio, where: n is a positive integer.